1. Field of the Invention
The present invention relates to a motor-driven power steering device for applying assistive steering torque produced by an electric motor to a steering mechanism, thereby reducing the manual steering forces required to steer a motor vehicle.
2. Description of the Relevant Art
Conventional motor-driven power steering devices employing electric motors for generating assistive steering torque are disclosed in Japanese Laid-Open Patent Publications Nos. 60(1985)-25853 and 59(1984)-50864 (corresponding to U.S. Pat. No. 4,415,054 patented Nov. 15, 1983), for example. The disclosed motor-driven power steering devices have a ball screw mechanism including a screw shaft comprising a rack shaft of a rack-and-pinion steering gear mechanism, with a helical groove defined in an outer peripheral surface thereof, a nut disposed over the screw shaft and having a helical groove defined in an inner peripheral surface thereof, and balls received in the helical grooves of the screw shaft and the nut.
In the former Japanese Patent publication, a speed reduction gear is integrally formed with the outer periphery of the nut of the ball screw mechanism, and a pinion meshing with the speed reduction gear is coupled through a clutch to a motor which produces assistive torque. Therefore, the torque of the motor is transmitted through the clutch, the pinion, and the speed reduction gear to the nut, and then is converted by the ball screw mechanism to axial linear motion applied to the rack shaft.
According to the latter Japanese Patent publication, the rotor of a motor is fixed to the outer periphery of the nut of the ball screw mechanism. Therefore, the motor is directly coupled to the ball screw mechanism.
The motor-driven power steering devices employing the ball screw mechanisms as described above suffer a small frictional loss, and hence have a high power transmission efficiency which leads to smooth power steering operation.
However, the above motor-driven power steering devices have been liable to fail to operate smoothly and also to have a durability problem since the rack shaft has a helical groove and serves as the screw shaft of the ball screw mechanism. More specifically, an axial or shearing load is applied to the rack shaft from wheels through knuckle arms and tie rods. When the vehicle is steered, the steering force is imposed on the rack shaft from the pinion shaft, and assistive torque is also applied to the rack shaft from the motor via the ball screw mechanism. Therefore, the rack shaft tends to flex under a bending moment, and to vibrate slightly in a radial direction due to movement of the meshing rack and pinion.
Generally, the ball screw mechanisms have an extremely low margin for shearing loads or stresses. When the screw shaft flexes or vibrates, the helical groove of the nut is apt to be damaged, and/or undue stresses act on the balls riding in the helical grooves. The balls under such undue stresses cannot rotate smoothly, with the results that the motor torque will not be transmitted efficiently, the steering feeling of the driver may be impaired, and/or the durability of the motor-driven power steering system will be lowered.
With the rack shaft helically grooved on its outer periphery, the mechanism for transmitting assistive torque cannot easily be maintained or serviced since it is troublesome to detach such mechanism. Lack of interchangeability of the grooved rack shaft and the rack shaft of an existing manually operated steering device results in an increased cost of the power steering devices, and makes it difficult to convert existing manually operated steering devices into motor-driven power steering devices.
Where a ball screw mechanism is employed in a motor-driven power steering device, the power transmission efficiency is higher as the lead angle of the helical groove on the screw shaft is larger. Inasmuch as the lead angle is determined by the magnitude of the output torque of the motor, if the motor used has a torque capacity greater than a certain torque level, then it is better to reduce the diameter of the screw shaft in a manner commensurate with the torque capacity of the motor for achieving a higher power transmission efficiency.
With the aforesaid conventional motor-driven power steering devices, however, the diameter of the screw shaft could not be reduced beyond its range providing desired mechanical strength since the rack shaft used as the screw shaft is helically grooved and subject to large axial and shearing loads. Therefore, the power transmission efficiency of the conventional motor-driven power steering devices is relatively low. It has also been difficult to improve characteristics of the power steering devices at the time of returning to the neutral position.
Additionally, the helical groove provided on the axially movable rack shaft over a certain length fails to provide a seal between the rack shaft and its housing, thus allowing dust to enter and rust to be formed in the ball screw mechanism.